The present invention relates to a developing device to develop an electrostatic or a magnetic latent image with two-component developer in which magnetic carrier particles and toner particles are mixed in an electrophotographic copier, and alternatively the present invention relates to a developing device to develop an electrostatic or a magnetic latent image with one-component developer composed of non-magnetic toner in an electrophotographic copier.
Conventionally, a magnetic brush type developing device using two-component developer is frequently applied to an electrophotographic copier. This developing device includes a magnetic roller composed of a magnet having a plurality of magnetic poles, and a cylindrical developing sleeve which is rotatably supported. Magnetic carrier particles to which toner particles are deposited are held on the surface of this developing sleeve so that the magnetic carrier particles are conveyed to a developing region for development. In this developing device, triboelectric charging can be relatively easily controlled, so that the toner particles are less susceptible to coagulation, and excellent bristles can be provided in the magnetic brush. The surface of an image carrier has desirable frictional properties, and cleaning can be sufficiently conducted on the surface of the image carrier. Moreover, this developing device is also suitable for non-contact development in which a developing operation is conducted under the condition that the image carrier surface is not contacted. For the reasons described above, this developing device is frequently used although the amount of toner particles must be controlled with respect to the amount of carrier particles. However, for the contact developing system in which the image carrier surface is rubbed by the magnetic brush, a developer composed of magnetic carrier particles of which the diameter is several tens to several hundreds .mu.m and non-magnetic toner particles of which the average diameter is about 10 .mu.m, is conventionally used. Therefore, the toner and carrier particles are so coarse that fine lines and small spots can not be reproduced, and delicate contrast can not be reproduced either. Accordingly, it is difficult to provide an image of high quality. This developing device has the problems described above. In order to solve the problems, a large number of countermeasures have been taken, for example, the carrier particles are coated with resin, and the magnet used for the developer carrier is improved. Although many efforts have been made, images of sufficient quality have not been provided yet. Therefore, it is necessary to reduce the sizes of toner and carrier particles, in other words, it is necessary to make the toner and carrier particles more minute. However, when the average particle size of toner is not more than 20 .mu.m, and especially when the average particle size of toner is not more than 10 .mu.m, the following problems are caused.
(1) In the developing process, Van der Waals force relatively affects Coulomb's force. Therefore, toner particles deposit on the background, and fogging is caused. Therefore, even when a DC bias voltage is impressed upon the developer carrier, it is difficult to prevent the occurrence of fogging.
(2) It becomes difficult to control triboelectric charging of toner particles, so that the toner particles are susceptible to coagulation.
(3) On the other hand, when the dimensions of carrier particles are reduced, the carrier particles are deposited on the electrostatic portion on the image carrier. The reason is considered to be that the force of the magnetic bias is lowered and the carrier particles are deposited on the image carrier together with the toner particles. In this connection, when the bias voltage is increased, the carrier particles are deposited on the background of the image. When the particle size is reduced, the above effect becomes remarkable, so that clear images can not be provided. Therefore, it is not practical to reduce the size of toner and carrier particles.
A developing device using one-component developer composed of non-magnetic toner includes a cylindrical developing sleeve which is rotatably supported, the surface of which is rough, and toner is held on the surface of this developing sleeve so that the toner is conveyed to the developing region for development. This developing device is also used for non-contact development in which development is performed under the condition that the image carrier surface is not contacted. However, in this developing device, a developer composed of non-magnetic toner particles, of which the average particle size is about 10 .mu.m, has been conventionally used. As the toner particles are relatively large, it is difficult to provide images of high quality on which fine lines and delicate contrast are excellently reproduced. In order to solve the above problems, it can be considered to reduce the size of the one-component developer, however, the aforementioned problems (1) and (2) are caused in the same manner as with the toner in two-component developer.
In order to solve the problems, the following methods have been proposed:
One method has been disclosed in Japanese Patent Publication Open to Public Inspection No. 223467/1984, applied by the inventors of the present invention, in which a control electrode to control airborne toner particles is provided in the development region, and development is performed in an oscillating electric field generated when a bias voltage having an AC voltage component is impressed.
Another method has been disclosed in Japanese Patent Publication Open to Public Inspection Nos. 131878/1991, 131879/1991 and 115264/1992, in which a plate-shaped member is disposed upstream of the developing region, and an AC bias voltage is impressed upon the plate-shaped member so that a toner cloud can be generated.
Still another method has been disclosed in Japanese Patent Publication Open to Public Inspection No. 94368/1989 in which a smoothing member is provided between the center of a development region and a restricting member to restrict the thickness of a developer layer, and a DC bias voltage is impressed upon this smoothing member, the polarity of which is reverse to the charging polarity of toner particles.
However, in the former method, when the control electrodes are used, they are stained and their electric field is oscillated. Moreover, it is difficult to maintain the control electrodes in alignment. Therefore, uneven density is caused, so that images of a predetermined level of quality can not be provided.
Even when the plate-shaped member is provided, the following problems are caused: the phase of the AC bias voltage of the control electrode is not the same between the image forming body and the developing sleeve; toner staining tends to occur since an AC electric field is generated in the upstream portion of the plate-shaped member; and further sufficient function is not provided so as to introduce the generated toner cloud into the developing region.
In the latter method, the bias voltage is impressed upon the smoothing member, the polarity of which is reverse to the charging polarity of toner particles, so that toner is deposited on the smoothing member. When the toner is adhered onto a photoreceptor surface, images are stained. Moreover, the oscillating conditions of the developer are changed, so that development is affected.